SUSCEPTIBILITY TESTING OF STENOTROPHOMONAS-MALTOPHILIA TO CARBAPENEMS

The susceptibility of 20 clinical isolates of Stenotrophomonas maltoph ilia to the carbapenems imipenem and meropenem was investigated by var ious methods. S. maltophilia appeared sensitive to meropenem but resis tant to imipenem by disc testing in Iso-sensitest agar. Agar dilution MICs were performed using Iso-sensitest agar and with incubation under three sets of atmospheric conditions. MICs of meropenem were consider ably lower than those of imipenem; this effect was maximal after incub ation in 5% CO2 when the MIC of meropenem was approximately 64 times l ess than that of imipenem. Induction experiments showed that both carb apenems could induce production of the L1 carbapenemase, However, disc approximation tests showed that imipenem could induced resistance to meropenem. partially stably derepressed mutants were readily selected in vitro. We conclude that, although S. maltophilia may give large zon es of inhibition to meropenem on disc testing, resistant mutants are r eadily selected and therefore standard sensitivity tests may be poorly predictive of clinical outcome of treatment of S. maltophilia infecti ons with meropenem.